1. Field of the Invention
The present invention relates to a cell shaping device, a communication system having the device, and a shaping method, and more particularly to a cell shaping device for ATM (Asynchronous Transfer Mode) cell in the ADSL (Asymmetric Digital Subscriber Line), a communication system having the device, and a shaping method.
2. Description of the Prior Art
The ADSL involves a service of best effort type, and the rate that can be acquired may significantly vary each time of linking up. The service of best effort type represents the communication form in which a certain type of service (e.g., transmission band) is not guaranteed in a network. Notwithstanding the best efforts as the overall network, the service may not be guaranteed from an end-to-end viewpoint, and provided at all in the worst case. In the ADSL, as one example, the service having the down-stream maximum speed of 12 Mbps is allowed up to the speed of 12 Mbps at maximum, but may be performed at reduced speed due to effects of environment and the like, whereby the speed of 12 Mbps is not always guaranteed.
For example, in the ISDN (Integrated Services Digital Network) that has been widely introduced into Japan, the time division multiplexing is employed for the transmission system. The ADSL has a property that it is very prone to degrade by the noise produced from the ISDN due to the transmission characteristics. Therefore, the acquired rate may be enhanced or degraded under the noise condition depending on the service state of the ISDN. At present, there are various services of 12 Mbps, 8 Mbps and 1.5 Mbps in the ADSL in Japan, but the actual acquired rate greatly varies depending on the type of service, and the connection mode.
Though the ADSL equipment applied in this system has a function of converting the data format from the ATM cell to the ADSL superframe, some ATM cells maybe discarded if data is passed from the ATM cells beyond the acquired rate of the ADSL, because the ADSL layer does not have a permissible capacity for passing all the data. Since the data to be sent is divided into minute cells having a data length of 53 bytes in the ATM, if the intermediate cells are discarded, the original data may not be restored, and the discarded cells must be retransmitted. Hence, a phenomenon occurs that the acquired rate of the ADSL is high but the throughput of data is slower.
Hence, in the actual operation, it is required that the transfer interval of cell is adjusted so that the ATM cells may not be discarded or overflowed. To do this, a technique of the ATM cell shaping is used.
One example of the conventional cell shaping device employing this ATM cell shaping will be described below. FIG. 7 is a block diagram showing one example of the conventional cell shaping device. Referring to FIG. 7, the cell shaping device is composed of a shaping portion 6 making up the ATM shaping device, an ADSL signal conversion portion 8 and a link information detecting portion 1 making up the ADSL communication device, and a setting portion 5.
For the sake of convenience, FIG. 7 includes an ADSL modem 9 provided in customer and an ATM network 7. The ATM network 7 is connected to the shaping portion 6, and the ADSL model 9 is connected to the ADSL signal conversion portion 8.
The setting portion 5 sets up a predetermined shaping value in the shaping portion 6. The link information detecting portion 1 detects the ink information, namely, acquired rate information, from the ADSL signal conversion portion 8.
In the conventional ATM shaping device, the shaping setting by the setting portion 5 and the detection of the link information by the link information detecting portion 1 are performed separately, as shown in FIG. 7. Accordingly, it is common that the setting portion 5 sets up a certain shaping value in the shaping portion 6 for each ADSL service.
At this time, if the setting portion 5 sets up the shaping value below the acquired rate of ADSL in the shaping portion 6, the total band of ADSL at the acquired rate can not be employed for the data transfer, so that the throughput of actual data is slower. In the inverse case, the throughput of actual data is also slower, because the cells are discarded as previously described.
On the other hand, one technique for controlling the shaping value has been disclosed. This technique decides the shaping band, based on the flow rate measurement result of ATM cells (refer to patent document 1).
However, in the technique as described in patent document 1, the shaping band is decided based on the flow rate of cell, but can not be decided when the flow rate is zero as one example.
On the contrary, in this invention, the shaping value is decided based on the acquired rate at the time of linking up, and can be decided, irrespective of the flow rate.
Accordingly, the technique as described in patent document 1 is totally different in the constitution, operation and effects from this invention, and it is difficult to arrive at the invention from the technique as described in patent document 1.
[Patent document 1]
Japanese Patent Laid-Open No. 2003-8634 (paragraphs 0019, 0021 and FIG. 2)